Metallic ferromagnetism in a band model: Intra-atomic versus interatomic exchange

The relative importance of intra-atomic versus interatomic direct exchange in leading to ferromagnetism in metals is not well understood. Here we examine the interplay of these interactions in a tight-binding model with two orbitals per site. The interatomic exchange interaction J is found to always read to ferromagnetism, and its effect is enhanced by both on-site intraorbital (U) as well as interorbital ((U) over bar) Coulomb repulsions. For certain band fillings, and in particular for a quarter-filled band, the intra-atomic exchange interaction J(0) by itself can also lead to ferromagnetism. However, in most cases the magnitude of J(0) required appears to be unphysically large. For other band fillings, J(0) does not lead to ferromagnetism even if infinitely large. In the presence of nearest-neighbor exchange J, J(0) will either enhance or suppress the tendency to ferromagnetism depending on the band filling. These results are used to interpret the occurrence of ferromagnetism in the transition-metal series. and it is concluded that intra-atomic exchange is not likely to play a significant role in the ferromagnetism of Ni or in Ni-Cu and Ni-Zn alloys. Overall our results support the conjecture that intraband interatomic direct exchange is the dominant interaction giving rise to ferromagnetism in metals, and that the essential physics of metallic ferromagnetism is contained in a single-band model without orbital degeneracy. [S0163-1829(97)01241-1].